Tissue factor is increasingly considered to be the main initial stimulus for blood clotting in normal hemostasis and likely plays a major role in the pathological thrombosis associated with neoplasia, atherosclerosis and inflammatory disease. Human plasma lipoproteins contain a potent inhibitor of tissue factor initiated coagulation that we call the Lipoprotein Associated Coagulation Inhibitor (LACI). Plasma LACI consists of primarily two forms: a 34 kDa form associated with low density lipoproteins and a 40 kDa form which is a LACI/apolipoprotein A-II mixed disulfide heterodimer associated with high density lipoproteins. By an unknown mechanism, plasma LACI antigen increases from two to ten fold following intravenous heparin, and this increase is almost entirely in the 40 kDa form. Our long term objective is to determine the mechanism and significance of LACI's associations with lipoproteins and heparin. More specifically, we intend to answer the following questions: Does LACI bind to lipoprotein lipid or protein? Does LACI contain hydrophobic modification? Is LACI a proteoglycan? Does LACI's association with lipoproteins affect its function or half life in plasma? Does LACI's association with apo A-II determine its binding specificity to lipoproteins or its half life in plasma? Is LACI bound to glycosaminoglycans in vivo? Where is LACI localized in normal and pathological tissues? How do plasma LACI levels relate to clinical diseases, especially diseases of coagulation, such as thrombosis or diseases of lipid metabolism? These studies will be performed using LACI purified from plasma or cultured cells and polyclonal and monoclonal antibodies to LACI. Various biochemical and molecular biological techniques will be employed including protein purification, high pressure liquid chromatography, metabolic labeling of cultured cells, enzymatic and chemical digestions of purified proteins, deletion and site directed mutagenesis, binding studies with cultured cells and isolated lipoproteins and immunoprecipitations.[HRt] These experiments are designed to provide new information concerning the biochemistry and physiology of LACI. The results should be of interest not only to scientists studying coagulation but to those considering the possible clinical importance and use of LACI and to those interested in lipoprotein assembly and metabolism.